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Microwave gas-solid reactivity in industrially relevant systems

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FLORIDA STATE UNIVERSITY
COLLEGE OF ARTS AND SCIENCES
MICROWAVE GAS-SOLID REACTIVITY IN INDUSTRIALLY RELEVANT SYSTEMS
By
ANTHONY FERRARI
A Dissertation submitted to the
Department of Chemistry and Biochemistry
in partial fulfillment of the
requirements for the degree of
Doctor of Philosophy
2015
ProQuest Number: 10000648
All rights reserved
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ProQuest 10000648
Published by ProQuest LLC (2016). Copyright of the Dissertation is held by the Author.
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Anthony Ferrari defended this dissertation on November 12, 2015
The members of the supervisory committee were:
Albert Stiegman
Professor Directing Dissertation
Vincent Salters
University Representative
Greg Dudley
Committee Member
Susan Latturner
Committee Member
John Dorsey
Committee Member
The Graduate School has verified and approved the above-named committee members, and
certifies that the dissertation has been approved in accordance with university requirements.
ii
I dedicate this to my fiancée Sara and my family back home in Maine. They have all helped give
me all the motivation I needed throughout this process.
iii
ACKNOWLEDGMENTS
I would like to acknowledge Dr. Al Stiegman for not only allowing me to explore almost
any direction of microwave chemistry that I could dream of, but also guiding me away from
directions that were sure to fail. Dr. Stiegman always greeted me with positivity even during
periods of less than desirable results. He has always been a great professor, researcher, and he
has always stood by me whenever I needed guidance.
I would like to thank all of the staff in the machine shop and glass shop. Specifically I
would like to thank Tom Dusek from the glass shop. Tom routinely put up with our wild
expectations and dreams of complex apparatuses. The amazing part about Tom was he would
routinely create said apparatuses. Without him the custom microwave experiments would
literally be impossible. He was always a great sport and would constantly help us achieve our
goals.
Lastly, I would also like to take the time to thank the various members of the Stiegman,
and Dudley Group especially Dr. Jake Hunt, and more recently Mitch Bogle. I have always tried
to learn everything I can from each student I have interacted with. Each interaction has helped
me problem solve, and move forward in whatever project I have encountered.
iv
TABLE OF CONTENTS
List of Tables ............................................................................................................................... viii
List of Figures ................................................................................................................................ ix
Abstract ......................................................................................................................................... xii
1.
INTRODUCTION ...................................................................................................................1
1.1
1.2
Microwave Background Information .............................................................................1
1.1.1
Microwave Heating ............................................................................................1
1.1.2
Microwave Heating of Carbon and Solids .........................................................3
1.1.3
Single Mode System .........................................................................................4
Experimental Design .....................................................................................................5
1.2.1
Physical Properties of Carbon Material ............................................................5
1.2.2 Reaction Conditions ...........................................................................................8
2.
MICROWAVE-SPECIFIC EFFECTS ON THE EQUILIBRIUM AND
THERMODYNAMICS OF GAS-SOLID REACTIONS .......................................................9
2.1
Background ....................................................................................................................9
2.1.1
2.2
Common Carbon Reactions with Steam ............................................................9
Experimental ................................................................................................................10
2.2.1
Steady State Experimental Design
2.2.2
Dielectric Measurements ...............................................................................11
2.2.3
G.C. Analysis ...................................................................................................12
v
..............................................................10
2.3
2.4
3.
4
Results ..........................................................................................................................12
2.3.1
Establishing the Equilibrium...........................................................................12
2.3.2
Probing the Power Threshold..........................................................................16
2.3.3
Equilibrium Overview ...................................................................................17
2.3.3.1
Steam-Carbon Reaction ..................................................................18
2.3.3.2
Boudouard Reaction .......................................................................22
2.3.3.3
Water-gas-shift Reaction ................................................................24
2.3.3.4
Carbon Hydrogen Reaction ............................................................27
Discussion and Conclusions .......................................................................................29
TEMPERATURE MEASUREMENTS IN ORGANIC MICROWAVE SYSTEMS ..........37
3.1
Introduction ..................................................................................................................37
3.2
Results and Discussion ................................................................................................38
3.2.1
Reflux Conditions ...........................................................................................38
3.2.2
Stir Bar Effects ................................................................................................39
3.2.3
Nucleation Site Effects ...................................................................................44
3.3
Experimental Section ...................................................................................................45
3.4
Conclusions .................................................................................................................46
A MECHANISTIC STUDY OF SOLID-GAS MICROWAVE ENHANCED REACTIONS:
NITROUS OXIDE CONVERSION OVER GRAPHITE .............................................................48
4.1
Introduction ..................................................................................................................48
vi
4.2
Carbon-Nitrous Oxide Reaction ..................................................................................49
4.3
Results and Discussion ................................................................................................52
4.4
5.
4.3.1
Product Distribution ........................................................................................53
4.3.2
Reaction Kinetics ............................................................................................54
Conclusion ..................................................................................................................56
CONCLUSIONS ...................................................................................................................57
REFERENCES ..............................................................................................................................59
BIOGRAPHICAL SKETCH .........................................................................................................63
vii
LIST OF TABLES
Table 2.1 Equilibrium Constants and Thermodynamic Parameters for the Steam –Carbon ........19
Table 2.2 Equilibrium Constants and Thermodynamic Parameters for the Boudouard ...............22
Table 2.3 Equilibrium Constants and Thermodynamic Parameters for the Water-Gas-Shift .......25
Table 2.4 Equilibrium Constants and Thermodynamic Parameters for the Carbon-Hydrogen ....27
Table 2.5 Thermodynamic Parameters for Thermal and Microwave Gas-Carbon .......................33
Table 3.1 Difference in Temperature for Solvents and Parameters Explored ..............................46
viii
LIST OF FIGURES
Figure 1.1 Diagram of conventional microwave field ......................................................................1
Figure 1.2 Solid absorbing material in a solution of non-absorbing solvent ....................................3
Figure 1.3 The recombination of charges visualized in a microwave field. .....................................4
Figure 1.4 Diagram of a single mode system alongside a multi-mode microwave reactor. .............5
Figure 1.5 SEM image of the graphite used in the experiments .......................................................6
Figure 1.6 Energy dispersion spectrum showing a) the elemental composition of the graphite
used in the experiments and b) impurities introduced during analysis. .........................7
Figure 1.7 The real (blue) and imaginary (red) components of the dielectric constant and the
loss tangent (green) of graphite used in these experiments. ...........................................8
Figure 2.1 Schematic of closed cell used to acquire equilibrium data under microwave
conditions. ......................................................................................................................11
Figure 2.2 Four independent determinations of temperature, pressure and composition at
approximately 5 minute intervals after equilibrium is established in the system (1 g
graphite, 200 W irradiation, 100 minutes to reach equilibrium). ...................................14
Figure 2.3 Pressure, temperature and equilibrium constants show that an equilibrated system
returns to the same equilibrium value for the steam carbon reaction after perturbation
by the addition of CO to the system. ..............................................................................15
Figure 2.4 Equilibrium percent compositions, as a function of the temperature under microwave
irradiation. ......................................................................................................................18
Figure 2.5 The equilibrium constants as a function of temperature for the microwave (blue) and
thermal (red) steam-carbon reaction. ..............................................................................20
ix
Figure 2.6 Van’t Hoff plots for the microwave driven equilibria present in the steam-carbon
(H2O+C) system.............................................................................................................21
Figure 2.7 The equilibrium constants as a function of temperature for the microwave (blue) and
thermal (red) Boudouard reaction. .................................................................................23
Figure 2.8 The equilibrium constants as a function of temperature for microwave (blue) and
thermal (red) water-gas-shift reaction. ...........................................................................26
Figure 2.9 Equilibrium composition of the reactants (H2O) and products (CO, H2) of the
steam-carbon reaction as a function of temperature under microwave (solid) and
thermal (dashed) conditions. ..........................................................................................30
Figure 3.1 Experiments run with no stirring present under reflux conditions, at 75 W for 3
minutes. The boiling points of each solvent have been plotted for clarity. ....................39
Figure 3.2 Small stir bar (10 x 3 mm) with stirring on under reflux conditions at a fixed wattage
of 75 W for 6 minutes.....................................................................................................40
Figure 3.3 Medium stir bar (25 x 5 mm) experiment with stirring on under reflux conditions at a
fixed wattage of 75 W for 6 minutes. .............................................................................41
Figure 3.4 Ethanol without a stir bar plotted against a stirred experiment, the boiling point was
also shown for 3 minute run. ..........................................................................................42
Figure 3.5 Methanol without stirring plotted against a stirred experiment, the methanol is much
more chaotic when compared to the ethanol. .................................................................43
Figure 3.6 Small stir bar experiment for different solvents over 3 minute period. .........................43
Figure 3.7 Side angle of superheated IPA during a typical run, this image was taken using the
PTFE tube instead of the typical quartz vessel to obtain the image. ..............................45
x
Figure 4.1 (a)Hypothesized mechanism for microwave enhancement of the steam-carbon
(shown in black) and Boudouard (shown in red) reactions. (b) proposed mechanism
for increased CO generation during Boudouard reaction. ..............................................48
Figure 4.2 Hypothesized microwave effects the N2O-carbon reaction due to space-charge
recombination and selective activation of the surface oxide. .........................................51
Figure 4.3 Temperature of graphite and nitrous oxide reaction under 30 W of microwave
irradiation. ......................................................................................................................52
Figure 4.4 Composition of product gas stream as a function of temperature: comparing the
percentage of product species between the two processes. ............................................53
Figure 4.5 Arrhenius plot of the thermal and microwave reactions. .............................................55
xi
ABSTRACT
Gas-Solid reactivity was extensively studied throughout the early 20th century and the
kinetics of these systems have become well established and well understood. Recently,
microwave active materials (conductive or magnetic materials that absorb microwave irradiation)
have been shown to produce increased reactivity in a significantly different way when compared
to conventional heating. Many of these active materials can be used to improve reactivity in
industrially relevant gas-solid systems. The possible rate enhancement of these systems can be
measured by using reaction kinetics, and the kinetic rates can be compared to the previously
studied, well established, thermal measurements. By understanding the difference between
microwave and conventional heating we may better predict which systems would be ideal
candidates for increased reactivity.
The steam–carbon reaction, which produces synthesis gas (H2 + CO) is the principle
reaction in a gasification process that produce energy. This reaction has been measured
extensively in the past and could be an ideal reaction to become enhanced using microwave
irradiation.
C + H2O  H2 + CO
At 131 kJ/mol this endothermic reaction uses carbon as its microwave active material. This solid
material can be any form of carbon (activated carbon, graphite, coal etc.) and it selectively heats
in a microwave reactor. This reaction was shown to have a large difference in apparent activation
energies and kinetic rates when compared to the thermal rates and energies. By using an
Arrhenius plot, the apparent microwave equilibrium constants were calculated at various
wattages and shown to be lower when matched against comparable temperature ranges of the
conventional thermal reactions. The enthalpy and entropy of the systems were then calculated to
xii
give an effective thermodynamic value to describe the energy differences. Not only was the
reaction more efficient in the microwave, but the microwave composition of the product gases
included less CO2, which would be produced from a water gas shift side reaction. These findings,
of a system that produces less side products at lower temperatures, are evidence that microwave
gas-solid reactions could provide unique chemistry that should be applied to more industrially
relevant systems.
Probing the mechanisms of these results was done by using a nitrous oxide and carbon
system to observe the compositional difference in reactivity.
2C + 2NO2  N2 + 2CO2
The interfacial polarization of the carbon is understood to be the method of heating in a
microwave reactor. Electron hole pairs are created as the charges separate and become trapped at
grain boundaries across the surface of the material. These electron hole pairs create an active site
on the surface that helps facilitate reactivity and sometimes leads to different compositional
makeup of product gases. Probing this mechanism was important to help describe which systems
would
be
good
candidates
to
study
xiii
in
further
research
endeavors.
CHAPTER 1
INTRODUCTION
1.1 Microwave Background Information
1.1.1 Microwave Heating
Microwaves consist of an electric field and a magnetic field component. These two fields
oscillate perpendicular to one another, resulting in electromagnetic radiation (EMR).
The
radiations frequencies range between 0.3 GHz and 300 GHz respectively. It is these specific
ranges that are defined as microwaves on the electromagnetic spectrum. For microwave heating
devices the frequency is generally fixed at 2.45 GHz or a wavelength of 12.2 cm to avoid
interference with communication devices which operate in this region of the spectrum.
Figure 1.1 Diagram of a conventional microwave field. The electric field (red)
oscillates perpendicular to the magnetic field (blue). The wavelength is 12.2 cm
or 2.45 GHz.
1
The heating of a solid material or a solution in a microwave field occurs through a
dielectric loss process. Dielectric loss processes are known as relaxation processes, which differ
from resonance processes in that there is no discrete absorption of energy to generate an excited
state. The underlying mechanism by which these loss processes occur differs in solid materials
and molecular solutions. In solid materials loss processes that lead to heating can be grouped into
two general categories: conduction and Debye-type loss.2 Conduction loss mechanisms occur
through the movement of charge, which can be electrons or ions. The generation of heat occurs
when the charge recombination is out of phase with the oscillating field. Among the most
significant dielectric loss mechanism for heating of carbon is interfacial polarization which
generates heat from electron hole pairs becoming trapped on the surface of the material which
inhibits their ability to recombine.1 For microwave heating of molecules in solution, the dielectric
relaxation process that occurs is based on theory developed by Debye. For heating to occur, the
molecules must have a dipole moment. The electromagnetic field couples to the dipole moment
causing it to align with the oscillating electromagnetic-field. The molecules oscillate but do not
remain in phase with the oscillations of the electric field due to collisional interactions with the
medium. These collisional interactions result in the generation of heat in a loss process that is
often viewed as frictional.2
The difference between conventional heating and microwave heating of a solid are
evident in the IR temperature images shown in figure 1.2. This figure shows a heating profile of
a solid material in an organic solution. The higher temperature is measured at the surface of the
absorbing solid material. The organic solution has a much lower surface temperature when
compared to the solid material in this specific example. Conventional convective heating would
result in the system being heated from the outside in, so that when steady-state is reached all
2
components, solids and liquid, would be at the same temperature. The microwave temperature
distribution can be far more complex and result in areas of hot spots which can be used to
specifically heat desired materials, as opposed to generating a uniform heating profile.
Figure 1.2 Solid absorbing material in a solution of non-absorbing
solvent .
1.1.2 Microwave Heating of Carbon and Solids
The volumetric heating of carbonaceous materials by microwave radiation at 2.45 GHz
generally proceeds efficiently, making carbon-based reactions an ideal candidate for microwave
enhancement.1,3 Studies of dielectric relaxation processes in different forms of carbon have
indicated that heat is produced primarily through space-charge (interfacial) polarization, which is
typical for many solid dielectric materials.3-7 Qualitatively, this loss mechanism arises from
charge carriers (electronhole pairs), which become trapped at the surface in defect sites,
impurity sites, and grain boundaries.4 The trapping process impedes the charge recombination,
thereby dephasing the charge transport from the oscillating electric field and resulting in
dielectric loss.
3
Figure 1.3 The recombination of charges visualized in a microwave field. The electric
field creates a charge separated state that begins to recombine. This recombination
process is shown here to be completely out of phase with one another.
The loss process is measured through the imaginary part of the dielectric constant, ’’, (eq 1.1)
with the magnitude of the loss often given through the loss tangent (eq 1.2).3
e = e ¢ - ie ¢¢
tan d = e ¢¢ e ¢
Eq
1.1
(1.1)
Eq
1.2
(1.2)
The magnitude of the loss, and hence the degree of heating, varies among different types of
carbon. Carbon activated at high temperature, for example, tends to heat extremely efficiently in
the microwave. It is important to note, however, that the dielectric loss tends to increase as a
function of temperature for activated charcoals.3
1.1.3 Single Mode System
Typically household conventional microwaves use a multimode system where the cavity
supports multiple standing waves which generate spatially distributed nodes of radiation due to
the interference pattern of the standing waves. This distribution tends to create a non-uniform
heating profile within the oven. This works well for quickly heating food, but is not desired
when designing a laboratory experiment. All of the experiments in the following study were
4
done using a single mode type of microwave. Figure 1.4 shows the difference between the two
systems. The wave guide of the single-mode system allows for more directed uniform
distributions that result in a more reproducible heating profile. The directed field interacts in the
cavity at a specific zone in the microwave reactor. For this reason, it is necessary to design
glassware or other apparatuses to hold the desired material directly in this uniform field.
Figure 1.4 Diagram of a single mode system alongside a
multi-mode microwave reactor.
1.2 Experimental Design
1.2.1 Physical Properties of the Carbon Material
The carbon source for the steam carbon reaction was commercially purchased graphite
powder (Fischer Grade #38); used as received, with an average BET surface area of 7.59 m 2/g
and average particle sizes between ~ 10 and 60 µm as observed by SEM microscopy (Figure
1.5a). The low surface area correlates to the lack of porosity observed in the SEM.
5
Figure 1.5 SEM image of the graphite used in the experiments at a) 2500
and b) 300 x magnification (regions where EDS was determined are
numbered in red).
The elemental composition of the graphite was determined from energy dispersive
spectroscopy (EDS) measurements. Close examination of the SEM images revealed that the
majority of particles appeared identical and had features characteristic of graphitic carbon
(Figure 1.5b, point 1). Also visible , were small bright particles that were distinctly different
from the other particles and were judged likely to be impurities (Figure 1.5b, points 2 and 3).
The elemental distribution of both of these components is shown in Figure 1.6. As suggested by
the morphology, point 1 (Figure 1.6a) is very pure graphite with C being >99 wt. % C and
containing small amount (<0.3 wt %) of Al, Si, Fe and Cu.
EDS of points 2 and 3 were
essentially identical their elemental composition was about 48.5 wt % C, 28% Fe and 22 % O
(Figure 1.6b).
6
Figure 1.6 Energy dispersion spectrum showing a) the elemental
composition of the graphite used in the experiments and b) impurities
introduced during analysis.
Dielectric measurements (as performed for the activated carbon) are presented in Figure
1.7. The largest values for the loss tangent occur where it peaks around 6.88 GHz, other maxima
are observed around 13.52 and 16.5 GHz. At the microwave excitation frequency of 2.45 GHz
the loss tangent of graphite was found to be 0.26 (±0.01). From the values of the real and
imaginary dielectric constants (10.14 (±0.29) and 2.62 (±0.16), respectively), a penetration depth
of 19.05 mm was calculated. The quartz vessel used in these experiments had a diameter of 24
mm at the graphite bed and was observed, as expected, to heat relatively uniformly with no
significant attenuation in the center of the sample.
7
Figure 1.7 The real (blue) and imaginary (red) components of the dielectric constant and
the loss tangent (green) of graphite used in these experiments. The yellow line indicates
the frequency (2.45 GHz) of the incident radiation from the microwave oven.
1.2.2 Reaction Conditions
Reactions were all carried out in a commercial CEM Discover microwave reactor in open
vessel configuration. To remove adsorbed species from the samples they were irradiated for 10
minutes at 200 W of microwave power and then evacuated. This cleaning procedure was
repeated twice more for a total of three cleaning cycles to remove any adsorbed water or surface
oxide species from the carbon. After the final cleaning cycle, the cell was evacuated and brought
to room temperature before slowly being vented to atmospheric pressure using inert gas. This
cleaning procedure is critical to create reproducible reactivity between experimental runs.
8
CHAPTER 2
MICROWAVE-SPECIFIC EFFECTS ON THE EQUILIBRIUM
AND THERMODYNAMICS OF GAS-SOLID REACTIONS
2.1 Background
2.1.1 Common Carbon Reactions with Steam
The reaction between superheated steam and carbon to produce synthesis gas (eq 2.1) is
part of the general category of gasification reactions used to obtain hydrogen from coal and other
carbon rich sources.8,9 Gasification reactions typically occur at temperatures ≥ 700 °C depending
on the carbon source, while industrial processes, such as coal gasification, run at much higher
temperatures (>1000 °C). High temperatures are required to drive the endothermic components
of the primary reactions and to obtain useful reaction velocities.21
(2.1)
Eq 2.1
Eq 2.2
(2.2)
Eq 2.3
(2.3)
Eq 2.4
(2.4)
Along with production of synthesis gas (eq 2.1), the reactions between carbon and high
temperature steam consist of a complex set of equilibria, which produce not only hydrogen and
carbon monoxide but also carbon dioxide through the water-gas-shift (WGS) reaction (eq 2.2),
carbon monoxide through the disproportionation of carbon and carbon dioxide (Boudouard
9
reaction, eq 2.3) and methane through the reaction of carbon and hydrogen (carbon-hydrogen
reaction, eq 2.4). The complexity of the equilibria and the fact that the primary steam carbon
reaction (eq 2.1) being very endothermic, means that the composition of the gases produced in
gasification will depend critically on the temperature and pressure of the reaction.
Because of the industrial importance of these reactions in the production of hydrogen for
direct use as a clean alternative fuel or for the production of hydrocarbons through the FischerTropsch process, the development of less energy intensive methods for driving these reactions is
desirable.10,11 As will be shown later, for the Boudouard reaction (eq 2.3), the use of microwave
radiation to heat the carbon results in a dramatic change in the observed thermodynamics of the
reaction.12 In particular, these thermodynamic changes result in an increase in the equilibrium
constant so that carbon monoxide becomes the favored product at 213 °C, compared to 643 °C
when using conventional convective heating. Given the importance of the general class of gas
carbon reactions in the overall scenario of energy production, it is useful to determine whether
the thermodynamic benefits that microwave heating imparts on the Boudouard reaction is
general for the other processes, in particular the endothermic steam-carbon reaction.
The
following study reports on the effect of microwave radiation on the fundamental
thermodynamics of the steam-carbon reaction (eq 2.1) and other equilibria that take place during
the gasification process.
2.2 Experimental
2.2.1 Steady State Experimental Design
All microwave reactions were carried out in a CEM Discover microwave oven under
conditions of fixed applied power. Equilibrium determinations were performed in a specifically
designed quartz apparatus, which can be centered in the cavity of the microwave. The steady-
10
state reactions were conducted in a quartz vessel of 24 mm I.D. with a fixed volume of 75.56 ml,
equipped with an absolute pressure gauge (Omegadyne PX409) and a 20 mm anti-reflective
coated germanium window (Edmund Optics, 8-12µm), Figure 2.1.
Figure 2.1 Schematic of closed cell used to acquire equilibrium data under microwave
conditions.
A high temperature infrared pyrometer (Omega OS554A-MA-6) was focused through the
germanium window to monitor the temperature of the carbon bed during the experiment. This
cell can observe pressures as low as 2 mTorr.
2.2.2 Dielectric Measurements
The sample measured must be formed as a toroidal cylinder, with the outer diameter the
same size as the cavity of the Damaskos solid sample holder, and the inner space the same
diameter as the center conductor of the Damaskos sample holder.
For measuring the
permittivity, the sample was placed in the holder, and its thickness and distance from the end of
11
the holder were measured. The S21 and S11 parameters were measured by the Damaskos
software, estimated values for the carbon (2.45 GHz: ɛ’ = 17.04 +/- 0.45 and ɛ” = 9.43+/- 1.60)
were imputed into the data reduction method “Nicolson Ross”.55 The initial value given by the
Nicolson-Ross method was then used as the initial guess in the “Eps From Transmission” data
reduction method, as per the Baker-Jarvis’s protocol.56
2.2.3 G.C. Analysis
The gas injections were done using 50 microliter injections into a HP 5890 series II gas
chromatograph that used a TCD (thermal conductivity detector). The samples were run using
Argon as the carrier gas isothermally at 100oC with injections 3 minutes apart to allow for
separation between injections. The injection port and detector were both set to 150oC. A Shin
Carbon micropacked column was used to separate the products due to the low molecular
weights.
2.3 Results
2.3.1 Establishing the Equilibrium
The results depend upon an accurate determination of the equilibrium constants under
microwave radiation, it is necessary to fully establish that equilibrium is attained. Using the static
reaction vessel (described above), charged with graphite and water, the pressure of the vessel and
the temperature of the carbon surface were monitored in situ during the course of the reaction.
Samples of the gas were extracted and analyzed by G.C. over the course of the reaction to
determine the amount of H2, CO, CO2 and CH4 present as a function of irradiation time. The
sample was initially irradiated at 200 W of applied power, yielding a surface temperature of 997
K with the establishment of equilibrium judged to occur when the pressure, temperature and
composition reached steady-state values. Multiple trials were carried out, and the time to reach a
12
steady state at 200 W was ~100 minutes in all cases. When the steady state was reached, the
atmosphere was analyzed four separate times (approximately once every 5 minutes) with the
partial pressures of the constituent species determined. The values of the partial pressures used in
calculating the equilibrium constants were the average of the four steady-state values. A typical
data set at 200 W applied power is shown below in Figure 2.2. The compositions at lower
temperatures were arrived at in the same system by reducing the applied microwave power and
allowing the system to reestablish equilibrium, which took roughly 10 minutes.
To calculate the equilibrium constants (Kp) for the various series of equilibria occurring
in the system, partial pressures (Pi) were calculated from the mole fraction (X) of the constituents
(i), determined from gas chromatography analysis, and the total pressure of the system,
Pi=XiPTotal. From the partial pressures Kp was then determined for each equilibria (eqn. 2.1-2.4).
2 
(2.1)
Water-gas-shift
(2.2)
= 
Boudouard reaction
(2.3)
= −
Carbon Hydrogen Reaction
(2.4)
2 2
 2
()2
2
4
2
(2 )
= −
Steam-Carbon
 2 
= 
The partial pressures of the system needed to be confirmed to be at equilibrium or they would
not have behaved as described. The Boudouard reaction was verified to be at equilibrium using a
similar experimental design that can be seen in figure 2.3.
13
Figure 2.2 Four independent determinations of temperature, pressure and
composition at approximately 5 minute intervals after equilibrium is
established in the system (1 g graphite, 200 W irradiation, 100 minutes to
reach equilibrium).
A separate experiment was performed to verify that the equilibrium, first, had been
established, and second, would return to the same value of the equilibrium constant when
perturbed by the addition of additional reactants or products. In this experiment (Figure 2.3), the
system was brought to equilibrium at 200 W of applied microwave power before being reduced
to 100 W, so as to not exceed the maximum safe pressure of the quartz equilibrium vessel.
The application of microwave radiation resulted in an initial decrease in temperature of
the carbon surface as the water is heated and subsequently vaporized. Upon achieving constant
temperature and pressure, the power was reduced to 100 W, and the system was allowed to reequilibrate for 30 minutes prior to product analysis by G.C. . The temperature of the graphite
14
surface was 516 °C at this power, and a value of Kp = 2.90 ± 0.21 was determined from analysis
of the constituent gases for the steam-carbon reaction (eq 2.1).
After the product gases were analyzed and the equilibrium constant was calculated, 5 mL
of CO (g) was injected into the closed vessel to perturb the system and shift the equilibrium
towards the reactants, leading to an increase in the total pressure of ≈ 70 mB (from 1110 to 1180
mB). Once the CO (g) was introduced, the system was allowed to re-equilibrate for 30 minutes,
and the composition was analyzed and the partial pressures of the products were used to
calculate the equilibrium constant of the perturbed system. The temperature of the graphite
surface, after given the appropriate amount of time to re-equilibrate, was 515 °C, and Kp was
determined to be 2.66 ± 0.17, which is, with experimental error, the same as determined prior to
the addition of CO, suggesting that under the conditions of the steam-carbon reaction,
equilibrium is established.
Figure 2.3 Pressure, temperature and equilibrium constants show that an equilibrated
system returns to the same equilibrium value for the steam carbon reaction after
perturbation by the addition of CO to the system.
15
2.3.2 Probing the Power Threshold
The generation of products from the steam-carbon and related gasification reactions is
expected to be dependent on the applied microwave power, in other words, there should be a
threshold power below which the reaction will not occur.13 In this particular study, which focuses
only on the thermodynamics of the reaction as a means to assess the magnitude of the microwave
effect, such a threshold will be related to whether any products can be produced and whether the
equilibrium can be attained in a reasonable period of time, which, in these experiments, was set
at 100 minutes. Since, given enough time, any system will ultimately reach equilibrium, this time
period, while somewhat arbitrary, reflected the practical experimental limitations of doing very
long term exposures in the microwave. To determine this threshold, a sample cell, charged as
described above with water and graphite, was systematically irradiated with applied powers,
starting at 10 watts and moving up in 5 W increments. At each power level, the system was
irradiated for 100 minutes, and the composition of the gas was determined. It was found that no
products gases (H2 and CO) were observed until 30 W of applied power, corresponding to a
carbon temperature of 373 °C, where trace amounts of H2 were detected. While this represents an
approximate minimum energy for the production of H2 and CO, it is well below a practical
threshold for the establishment of equilibrium in a reasonable period of time. It was found that at
90 W, while products were readily observed, the system was far from equilibrium, even after 240
minutes. Upon increasing the power to 100 W (491 °C), equilibrium was attained within 120
minutes. As such, all equilibrium determinations were carried out at ≥100 W, which was viewed
as the practical range for the establishment of equilibrium in our system.
16
2.3.3 Equilibrium Overview
The reaction of carbon with steam, which forms the basis of energy production processes,
such as coal gasification, has been studied experimentally under kinetic conditions using flowing
reactor systems.9,14-18 In general, the gas mixture produced from the reaction contain all of the
possible species, review the equilibria shown above (eq 2.1-2.4). The dominant species are H2
and CO, with the H2 coming from the steam-carbon (eq 2.1) and water-gas shift reactions (eq
2.2). The reaction of CO2 with carbon to produce CO (Boudouard reaction) does not contribute
appreciably until high temperatures are reached due to its high endothermicity. Similarly, though
for the opposite reason, the production of CH4 through the hydrogenation of carbon (eq 2.4) is
always a very minor pathway because at the temperatures at which gasification reactions are
typically run, the equilibrium lies to the left. In addition, the primary step in the reaction, which
is the hydrogenation of the graphite, is known to be extremely slow.12
The changes in the apparent thermodynamics of the reaction were proposed to be the
result of specific microwave effects on the mechanism from the interaction of the radiation with
the carbon surface. Equilibrium constants for the steam-carbon reaction system were determined
at five different power settings (100, 125, 150, 175, and 200W) that produced graphite surface
temperatures from 764 to 997 K. The distribution of species at equilibrium, as a percent of the
total composition, is shown in Figure 2.4 for each of the measured graphite surface temperatures.
17
Figure 2.4. Equilibrium percent compositions, as a function of
the temperature under microwave irradiation.
In figure 2.4, as the temperature increases, the amount of syngas, CO and H2, increases
steadily at the expense of the minor constituents, H2O, CH4 and H2O. This trend is consistent
with the known thermodynamics of the various equilibria. The strongly endothermic reactions,
the steam-carbon (eq 2.1) and the Boudouard (eq 2.3), have their equilibria shifted to the right
generating more CO and H2 as the temperature increase. Conversely, the equilibria of water-gasshift (eq 2.2) and carbon-hydrogen reaction (eq 2.4), shift to the left with increasing temperature
due to the exothermicity of those reactions, thereby contributing to the production of CO and H2
while depleting CO2 and CH4.
2.3.3.1 Steam-Carbon Reaction.
The steam-carbon reaction is the most industrially relevant of the gas-carbon reactions, as
it is used directly for the production of syngas. As such, it is of interest to determine whether a
18
significant microwave derived thermodynamic advantage will be realized for the steam-carbon
reaction. The equilibrium constants and values for the free energy of the steam-carbon reaction,
determined under microwave irradiation as a function of the temperature of the graphite, are
shown in Table 2.1. To provide a quantitative comparison between the thermochemical
properties of the microwave and thermal reaction, we have calculated the free energy change and
the equilibrium constant using contemporary thermodynamic data (Hf° and S°), temperaturecorrected using the appropriate heat capacities to match the temperature of the carbon surface
measured in the microwave experiments.
Table 2.1 Equilibrium Constants and Thermochemical Parameters for the Steam-Carbon
Reaction
Microwave
T(K)
T(K) Gasa
ΔG (kJ/mol)b
764(±4)
293
3.19(±.53)
-7.4(±1.0)
832(±3)
309
3.90(±.61)
-9.4(±1.0)
893(±2)
321
4.39(±.43)
-11.0(±.7)
949(±2)
331
4.95(±.50)
-12.6(±.8)
997(±2)
337
5.61(±.48)
-14.3(±.7)
Kp
ΔH (kJ/mol)
ΔS (J/Kmol)
15.2(±0.8)
29.5±(0.
1)
Thermal
Kp c
ΔG (kJ/mol)
764
0.03
23.5
832
0.16
12.8
893
0.66
3.12
949
2.07
-5.74
997
4.99
-13.33
T(K)
ΔH (kJ/mol)
ΔS (J/Kmol)
144.2
158.1
a.) Estimated using the ideal gas equation; b.) Calculated from ΔG = -RTlnKp, where
R is the ideal gas constant; c.) All tabulated thermodynamic quantities and
equilibrium constants were calculated from standard thermochemical data.
19
As the data shows, under microwave conditions the equilibrium lies significantly farther
to the right, favoring the production of synthesis gas at a much lower temperature than it does
under conventional thermal heating. A plot of the equilibrium constants, as a function of
temperature, for the microwave and thermal reaction is shown in Figure 2.5. The thermal
reaction has a rapid drop in the magnitude of the equilibrium constant as the temperature drops
below ~1000 K while the decrease is much more gradual under microwave irradiation.
Figure 2.5 The equilibrium constants as a function of temperature for the
microwave (blue) and thermal (red) steam-carbon reaction.
Because the enthalpy of gas-carbon reactions tends to vary only slightly with temperature
due to the small temperature-dependent heat capacities of the gas phase reactants, the enthalpy of
the microwave-driven reaction can be estimated from a Van’t Hoff plot of ln(Kp) versus 1/T.20
As can be seen in Figure 2.6, a linear relationship for the regression analysis was observed. The
value of H obtained from the slope of the plot was 15.2 (±.08) kJ/mol, while the value
20
calculated for the thermal reaction, in this temperature range, was 144.2 kJ/mol (Table 2.1).
Clearly, under microwave irradiation, the apparent thermodynamics of the reaction changes
dramatically. The microwave-driven reaction has a negative G at lower temperatures than the
thermal reaction due to its significantly smaller enthalpy, H, which will tend to be less than TS at lower temperatures. Conversely, the lower entropy means that as the temperature
increases, the thermal process will become more favorable much more quickly than the
microwave process as -TStherm > -TSmicro. Equating the thermodynamic relationship, G = HTS, for the two processes, the temperature at which both processes have the same value for G
is 1003 K; above that temperature, the thermal process is more favorable for producing CO and
H2, and below it, the microwave process dominates.
Figure 2.6 Van’t Hoff plots for the microwave driven equilibria present
in the steam-carbon (H2O+C) system.
21
2.3.3.2 Boudouard Reaction.
Because CO is produced as a part of the steam-carbon reaction, and CO2 is rapidly
produced from the water-gas shift reaction, the Boudouard reaction is one of the equilibria
present in the steam-carbon reaction. Due to its high endothermicity, this reaction only plays a
small role in the thermally driven steam-carbon process until very high temperatures are reached.
In the microwave, however, it was found to be much more exothermic, so it is likely to play a
more significant role by converting CO2 created in the water-gas-shift reaction back into CO.12
Table 2.2 Equilibrium Constants and Thermochemical Parameters for the Boudouard
Reaction
Microwave
T(K)
T(K)
Gas
ΔG (kJ/mol)b
Kp
a
764(±4)
293
2.97(±.38)
-6.9(±.8)
832(±3)
309
4.08(±.14)
-9.7(±.2)
893(±2)
321
5.17(±.37)
-12.2(±.5)
949(±2)
331
6.50(±.24)
-14.8(±.3)
997(±2)
337
7.83(±.25)
-17.1(±.3)
ΔH (kJ/mol)
ΔS (J/K mol)
27.0 (±.7)
43.1(±1.4)
Thermalc
Kp c
T(K)
ΔG (kJ/mol)
764(±4)
0.0048
34.12
832(±3)
0.050
21.12
893(±2)
0.29
9.458
949(±2)
1.2
-1.249
997(±2)
3.6
-10.43
ΔH (kJ/mol)
ΔS (J/K mol)
180.2
191.2
a.) Estimated using the ideal gas equation; b.) Calculated from ΔG = -RTlnKp, where
R is the ideal gas constant; c.) All tabulated thermodynamic quantities and
equilibrium constants were calculated from standard thermochemical data.
22
The free energy and associated equilibrium constants for the Boudouard reaction,
determined experimentally in the microwave and calculated from the thermodynamic tables over
the temperature range probed experimentally, are shown in Table 2.2. The free energy is
negative over the entire temperature range under microwave conditions while it becomes
negative only at 949 K and above for the thermal reactions.
Figure 2.7 The equilibrium constants as a function of temperature
for the microwave (blue) and thermal (red) Boudouard reaction.
The equilibrium constants for the two processes over the temperature range investigated
are shown in Figure 2.7. As with the steam-carbon reaction, the equilibrium under microwave
conditions favors the production of CO at significantly lower temperatures than are possible
thermally. From the Van’t Hoff plot (Figure 2.6), the value of the enthalpy for the microwavedriven reaction was determined to be 27.0 kJ/mol, which is more exothermic; from the free
energy and enthalpy, the entropy was found to be 43.1 J/mol. Both of these values are
significantly less than those determined for the thermal process.
23
2.3.3.3 Water-gas-shift Reaction.
The water-gas-shift (WGS) reaction is an integral part of the carbon steam reaction,
which accounts for production of CO2 in the product gas (eq 2.2). The reaction is weakly
exothermic and, unlike the other reactions in the steam-carbon system, carbon is not a reactant.
Instead, the reaction is thought to be catalyzed by the carbon and, under conditions typical of the
steam-carbon reaction, the equilibrium is rapidly attained. 9 Among the gas-carbon reactions, the
water-gas-shift reaction is not independent but can be written as the difference between the
steam-carbon and Boudouard reaction (i.e. rxn. 2.1-rxn. 2.3) so that the equilibrium constants are
simply related, (Ksc)(KBou)-1= KWGS. Thermodynamically, this means that the enthalpy of the
reaction is the difference in enthalpies, Hwgs=Hsc-HBou, and similarly, the entropy is
Swgs=Ssc-SBou.
For sake of completeness and comparison purposes, the values for the equilibrium
constants and thermodynamic parameters for the WGS shift reaction were calculated from the
experimental data, as was done for the other equilibria. The directly determined values (Table
2.3) were consistent with expected thermodynamic relationships to the steam-carbon and
Boudouard reactions described previously (Tables 2.1 and 2.2). As would be expected for an
exothermic reaction, the free energy increases with temperature and the equilibrium constant will
favor the products at lower temperatures. This trend is observed in both the thermal and
microwave reaction. Shown in Table 2.3, over the temperature range studied, the microwavedriven reaction equilibrium generally lies further to the right with the free energy becoming
negative at the low end of the temperature range studied.
24
Table 2.3 Equilibrium Constants and Thermochemical Parameters for the Water-Gas-Shift
Reaction
Microwave
T(K)
a
Kp
ΔG (kJ/mol)b
764(±4)
293
1.08(±.2)
-0.47(±1.1)
832(±3)
309
0.95(±.15)
0.35(±1.0)
893(±2)
321
0.85(±.1)
1.22(±.9)
949(±2)
331
0.76(±.08)
2.14(±.8)
997(±2)
337
0.72(±.06)
2.8(±.7)
T(K)
Kp c
ΔG (kJ/mol)
764
0.267
8.39
832
0.168
12.34
893
0.118
15.88
949
0.089
19.14
997
0.071
21.92
T(K)
Gas
ΔH (kJ/mol)
-11.4(±.4)
ΔS (J/K mol)
-14.1(±.2)
Thermal
ΔH (kJ/mol)
ΔS (J/K mol)
-36.0
-58.1
a.) Estimated using the ideal gas equation; b.) Calculated from ΔG = -RTlnKp, where
R is the ideal gas constant; c.) All tabulated thermodynamic quantities and
equilibrium constants were calculated from standard thermochemical data.
25
Figure 2.8 The equilibrium constants as a function of temperature
for microwave (blue) and thermal (red) water-gas-shift reaction.
The enthalpy of the water-gas-shift reaction under microwave conditions is -11.4 kJ/mol,
and the entropy is -14.1 J/mol. Clearly, the presence of the microwaves makes the apparent
enthalpy of the reaction more endothermic by 24.6 kJ/mol and the entropy more positive by 44
J/mol. The thermodynamic advantage provided by the microwaves is only realized at higher
temperatures where the small negative entropy causes TS< H. As the temperature decreases,
the free energy of the thermal reaction becomes more negative per degree temperature change
due to the large entropy. Equating the free energy expressions for the thermal and microwave
reaction, it is determined that at 559 K, both processes have the same free energy (-3.51 kJ/mol);
below that value the thermal process dominates.
26
2.3.3.4 Carbon Hydrogen Reaction.
The carbon-hydrogen reaction, which initially produces methane, is a minor component
of the carbon-gas reaction system. When our system attains equilibrium, small amounts of
methane are detected at concentrations that are steady-state over the course of the experiment,
and decrease with reaction temperature. Values for the equilibrium constant and G for the
carbon-hydrogen reaction were calculated (Table 2.4) and compared to the predicted values of
these quantities for the thermal reactions. The data suggest that, under microwave conditions, the
free energy is always positive and the equilibrium constant always lies to the reactant side over
the temperature range investigated. In contrast, the thermal reaction has negative values of G as
the temperature decreases, consistent with the exothermic nature of the reaction.
Table 2.4 Equilibrium Constants and Thermochemical Parameters for Carbon-Hydrogen
Reaction
Microwave
T(K)
T(K)
ΔG (kJ/mol)b
a
Kp
764(±4)
293
0.087(±.007)
15.5(±.52)
832(±3)
309
0.078(± .002)
17.7(±.13)
893(±2)
321
0.070(± .002)
19.8(±.21)
949(±2)
331
0.066(± .001)
21.4(±.16)
997(±2)
337
0.062(± .002)
23.1(±.25)
Gas
27
ΔH (kJ/mol)
-9.1(±.6)
ΔS (J/K mol)
-32.2(±.9)
Table 2.4-Cont.
Thermal
T(K)
Kp c
764
5.332
-10.63
832
1.914
-4.49
893
0.871
1.03
949
0.462
6.09
997
0.284
10.43
ΔG (kJ/mol)
ΔH (kJ/mol)
-79.7
ΔS (J/K mol)
-90.4
The enthalpy of the microwave-driven reaction, determined from the Van’t Hoff plot
(Figure 2.8) is -9.1 kJ/mol and the entropy, determined from the free energy and the enthalpy, is
-32.2 J/mol. The standard thermodynamic relationship (G=H-TS) predicts that in the
microwave, G will remain positive all the way down to 282.6 K, whereas for the thermal
reaction, free energy is negative below 881 K. This is an interesting result, as it means that the
reverse reaction, the dehydrogenation of methane, will be favored. In recent years, this
dehydrogenation reaction has been of some interest in the production of clean hydrogen fuel. 21,22
However, this result must be approached with some caution. The forward reaction of hydrogen
reacting with carbon is known to be extremely slow, with methane formation rates ranging from
10-16 to 10-12 mol g-1 sec-1 between 600 and 1100 K.19,23,24 As such, equilibrium for this reaction
may not have been fully attained. Careful scrutiny of the measured CH4 concentrations for the
data points used to determine the equilibrium constants across the temperature range indicates
that, within experimental error, a systematic increase in CH4 generation was not observed, which
suggests that the system is either at equilibrium or closely approaching it. As such, it is
28
reasonable to suggest that the direction, if not the absolute magnitude, of the microwave effect is
likely to be correct.
2.4 Discussion and Conclusions
The reaction of primary interest is the steam-carbon reaction, which is highly
endothermic and is used commercially to generate synthesis gas. As indicated in the results, there
is a clear shift in the equilibrium towards the desired syngas products under microwave
irradiation. A more general view of the effect of microwaves on the complex equilibria that
makes up the gasification reaction system can be estimated from the thermodynamic parameters.
In particular, the equilibrium composition in mole fractions of the total reaction system over a
broad temperature range can be approximated from the predicted values of the equilibrium
constants, in terms of mole fraction (K), obtained from the thermodynamic parameters (eq 2.5),
which are obtained experimentally for the microwave reaction and calculated for the thermal
reaction.

−

 =  − =  −
Eq
2.5
(2.5)
By using equation 2.5 for the temperature dependence of each of the equilibrium
constants in eq 2.1-2.4, the equilibrium expressions can be solved numerically for the mole
fractions of each constituent as a function of temperature. Figure 2.9 shows the approximate
composition of the reactant (H2O) and products (CO and H2) of the steam-carbon reactions as a
function of temperature for the microwave and thermal process. In this figure, in the microwave,
the consumption of water and generation of hydrogen and carbon monoxide occurs at
significantly lower temperatures than it does thermally—consistent with the more exothermic
nature of the reaction. These calculations predict that hydrogen will reach a maximum at 463 K,
29
whereas thermally it is expected to occur at 890 K. The calculations predict that, in general, less
CO will be produced under microwave radiation, with its production predicted to slowly
decrease over the temperature range, while under thermal conditions, it is on par with H2, with its
production peaking at 917 K. This arises because the lower temperatures lead the exothermic
water-gas-shift reaction to be more favorable, and CO is consumed. As the temperature goes up,
the water-gas-shift reaction becomes less favorable; however, the Boudouard reaction, which is
thermodynamically more favorable under microwave conditions but still endothermic, begins to
produce CO from CO2 at the higher end of the temperature range, yielding the predicted curve.
Figure 2.9 Equilibrium composition of the reactants (H2O) and products (CO, H2) of
the steam-carbon reaction as a function of temperature under microwave (solid) and
thermal (dashed) conditions. The vertical lines represent microwave thresholds for
product formation. The cyan line is the lowest temperature (power) at which any H2 is
observed, and the red line is the lowest temperature at which equilibrium could be
established in 100 min.
30
While these plots provide a good graphical comparison of the difference between
microwave and thermal reactivity for the steam-carbon system, the low temperature region of the
graphs is not accessible due to the threshold power limits of the microwave reactions. In short,
while the graph indicates that highest equilibrium concentration of H2 should be reached at 463
K, this is below the threshold where any product would be observed in the microwave. The
practical threshold temperatures are indicated by the vertical red lines in Figure 2.9; the lowest
temperature line (cyan) is associated with the lowest power at which any H2 is observed, and the
upper temperature line (red) is the lowest temperature (i.e., power) at which equilibrium could be
established in a reasonable period of time (100 minutes). All reported equilibrium data were
collected at or above the highest threshold. It is important to note, however, that when evaluating
the magnitude of any possible advantage of using microwaves to drive the steam-carbon
reaction, it will be necessary to perform the reaction under kinetic conditions with flowing
reactants. Under those conditions, parameters such as the power, flow rate and carbon mass (i.e.,
path length) can be varied to optimize product production.
The equilibrium constants and associated thermochemical parameters that were
determined for the steam-carbon process are important for understanding the magnitude and
direction (exothermic or endothermic) of the microwave-specific effect. What is indicated by the
enthalpy and entropy for the various constituent reactions of the process is that the microwave
effect is quite pronounced, in most cases differing significantly from what is predicted for the
conventional thermal reaction. However, the magnitude and direction of the apparent effects is
reaction dependent, with both increases and decreases in enthalpy and entropy observed.
A useful measure of the magnitude and direction of the microwave-specific effect can be
obtained from thermodynamic considerations. The enthalpy of the gas-carbon reactions can be
31
determined in the standard way from the standard enthalpies of the products and reactants at the
reaction temperature, T, (eq 2.62.8). For the thermal reaction, this is the difference between the
product and reactant, including the relevant stoichiometry factors (n) (eq 2.6).
é
ù
é
ù
DH thermal = ê å ni ( DH °T ) ú
- ê å ni ( DH °T ) ú
- ( DH °T )C
ë i
û product ë i
û reactants
(2.6)
Eq
2.6
(4)
é
ù
é
ù
DH microwave = ê å ni ( DH °T ) ú
- ê å ni ( DH °T ) ú
- ( DH °T + DH ° MW )C
ë i
û product ë i
û reactants
Eq
2.7
(2.7)
(5)
°
DH thermal - DH microwave = ( DH MW
)C
(2.8)
(6)
Eq
2.8
For the gas phase reactants and products, it is assumed that even though the average
temperature of the gas in the medium is much lower in the microwave experiment, the
temperature of the gas equilibrates rapidly with the carbon surface when the reaction takes place.
As such, the enthalpies are the same in both processes. It was also assumed, since carbon is the
only reactant that directly absorbs the microwaves, that the magnitude of the microwave effect
can be treated as a change in apparent enthalpy of the carbon. The enthalpy was written as the
sum of the thermal enthalpy and the enthalpy due to the microwave-specific contribution (eq
2.7). The magnitude of the microwave-specific enthalpy imparted to the carbon is the difference
between the microwave and thermal reaction enthalpy (eq 2.8), which for the steam-carbon
system is 129.1 kJ/mol. A similar analysis can be carried out for the entropy (Sther-Smw),
which, for the steam-carbon reaction is 128.6 J/mol. This value constitutes the apparent free
entropy induced in the carbon by the microwave radiation. Clearly, from a thermodynamic
standpoint, if the value obtained from equation 2.8 is positive, it appears as if extra heat (or
32
entropy) is being “stored” in the carbon, whereas if it is negative, it appears as though there is an
effective heat (or entropy) loss.
The key thermodynamic values for the thermal and microwave reactions and the
magnitude and direction of the microwave-specific effect are summarized in Table 2.5. The
microwave effectively increases the enthalpy and entropy of the carbon, thereby making the
overall reaction more exothermic and reducing the overall entropy.
Table 2.5 Thermodynamic Parameters for Thermal and Microwave Gas-Carbon Reactions
Reaction
Thermal
Microwave
Microwave Effect
Δ(H°)*C
(ΔS°)§C
29.5
129.1
128.6
27.0
43.1
153.2
148.1
-58.1
-11.4
-14.1
-24.6
-44.0
-90.4
-9.1
-32.2
-70.6
-58.2
ΔH
ΔS
ΔH
ΔS
(kJ/mol)
(J/Kmol)
(kJ/mol)
(J/Kmol)
Steam-Carbon
144.2
158.1
15.2
Boudouard
180.2
191.2
Water-Gas-Shift
-36.0
Carbon Hydrogen
-79.7
*
ΔHther-ΔHmicrowave ; §ΔSther-ΔSmicrowave
For the carbon-hydrogen reaction, which is inherently exothermic, the effect of the
microwave is very different; it makes the reaction more endothermic and increases the effective
entropy. The apparent effect on the carbon is that heat has been “removed” by the microwave.
Of course, the microwave is not literally adding or subtracting heat (or entropy) to or
from the carbon, above and beyond what exists, due to the temperature at which it is held and
which will be the same for both the thermal and microwave reaction at the same temperature.25
The observed microwave effect on the thermodynamic properties of the reaction must be due to
changes in the thermochemical kinetics of the forward and reverse reactions, which define the
33
position of equilibrium. Since the gas phase species don’t absorb microwaves, these
thermodynamic changes must necessarily arise from changes in reactivity of the carbon surface,
or species adsorbed on the surface, through interactions with the radiation.26,27 In a simple
fashion, the temperature dependence of the equilibrium constant and the forward and back rates
can be written in terms of free energy of activation.20
Kp = e
- DG o
RT
=
-G1*
RT
k1
ae
= 1 -G*
-1
k-1
a-1e RT
*
DG o = ( G1* - G-1
)
Eq(2.9)
2.9
(2.10)
Eq
2.10
In equation 2.9, the forward and reverse reactions have a free energy of activation
associated with them and a pre-exponential factor, a, which, for a gas-surface reaction, will itself
have a temperature dependence.20 From this relationship, it can be seen that free energy of the
reaction is equal to the difference in the activation free energy of the forward and reverse
reaction (eq 2.10). The effect of microwaves on the positions of the equilibrium can be
interpreted as arising from differential changes in the forward and reverse activation parameters:
free energy (G*) and selective heating (T, a). Since these are gas-solid reactions, the kinetics are
not described by a pair of elementary opposing reactions but involve adsorption and desorption
processes at the surface and reactions with active sites or other reactants on the carbon surface.
As such, both the forward and reverse processes will have thermo-kinetic parameters associated
with each discrete step in the mechanism, many of which can potentially be influenced by the
microwave radiation.
For the water-gas-shift reaction (eq 2.2), it is obvious that the selective microwave effect
on the equilibrium constant arises from different magnitudes of microwave acceleration of the
forward and reverse reaction. As discussed previously, the water-gas-shift reaction and its
34
thermodynamic parameters are obtained simply from the difference between the steam-carbon
(eq 2.1) and Boudouard (eq 2.3) reaction. Mechanistically, the forward reaction involves initial
oxidation of the carbon surface by water to produce hydrogen, with CO subsequently reacting
with the oxidized surface to yield CO2.9,28 As such, the observed microwave effect, in which the
enthalpy of the reaction was found to be more endothermic, arises because the reverse reaction is
simply the Boudouard reaction, which has a greater microwave-induced exothermicity (ΔH°MW)
than the forward, steam-carbon reaction, so the magnitude of the microwave effect of the watergas-shift reaction in Table 2.5 is (ΔH°MW)steam-carbon-(ΔH°MW)Boudouard=-24.6 kJ/mol. An analogous
analysis holds for the entropy.
For the carbon-hydrogen reaction (eq 2.4), the effect of the microwaves is to induce an
endothermic shift in the thermodynamics, which shifts the equilibrium to the left, favoring the
formation of H2 from CH4. The reverse reaction will produce clean hydrogen fuel from
hydrocarbons and, as such, is of significant interest. In fact, the reverse reaction is a key
component of the dry reforming of methane (CH4 + CO2 2CO + 2H2) over carbon, which has
been found to be accelerated by microwaves.29-33 The detailed mechanism of carbon-hydrogen
reaction has not been studied extensively, but the primary gas surface process for the forward
reaction is thought to involve sequential hydrogenation of the carbon surface. For graphite, this
takes place primarily at the edges of the basal plane. After a sufficient degree of hydrogenation,
methane—the initial product—is eliminated.9,24,34 The mechanism of the reverse reaction, which
involves dehydrogenations of the methane, is not as well understood. The reaction results in
deposition of carbon and involves a dehydrogenation and elimination of H2, the mechanism of
which is unclear but likely involves transfer of hydrogen to the carbon surface from which it is
subsequently eliminated. The effect of the microwaves on either of these reactions is difficult to
35
assess. Using our model, in which enhanced reactive sites are charge-separated sites on the
surface induced by the microwave radiation, it is reasonable to suggest that the hydrogenation
step, in both directions, might be accelerated, though possibly to different degrees. Alternatively,
the acceleration of the reverse reactions may arise from the interfacial interactions of the C-H
group with the microwave radiation, which may preferentially accelerate the elimination of H2
from the surface in the reverse reaction over CH4 in the forward reaction. Obviously, many other
factors may be at play in such a mechanistically complex reaction.
In conclusion, this study indicates that there is a significant microwave-specific effect on
the apparent thermodynamics of the reactions associated with the steam-carbon gasification
process. The net effect of the microwaves is not constant but depends on the specific reaction
and will depend on the differential effect that the microwaves have on the forward and reverse
reaction. From a practical standpoint, the most significant observation is that, under microwave
irradiation, the thermodynamics of the reaction favor the production of synthesis gas from the
steam-carbon reactions. In addition, the results also suggest that microwave radiation favors the
production of H2 from the carbon-hydrogen reaction. In both cases, the use of microwaves
reduces the temperature required to drive endothermic reactions, which produces
environmentally desirable hydrogen as fuel. In terms of fundamental science, the work supports
the hypothesis that, for heterogeneous reaction systems, microwaves not only selectively heat the
substrate but, beyond that, also affect primary thermo-kinetic steps of the process in order to
yield dramatically different equilibrium properties. This latter effect is proposed to be related to
the mechanism by which dielectric loss occurs through microwave interactions with carbon.
36
CHAPTER 3
TEMPERATURE MEASUREMENTS IN ORGANIC
MICROWAVE SYTEMS
3.1 Introduction
Correctly measuring temperatures in our gas-solid systems has proven to be a critical part
of the experimental design. At times in a closed system simply using convention temperature
recording techniques is impossible. In a microwave vessel even an open systems temperature
reading can become elusive. Knowing this fact, we looked into aiding the temperature reading
techniques of organic microwave systems. It turned out that a debate within the organic field had
already been in progress.
Microwave superheating has been discussed more and more in recent literature, and
although the quantity of the publications on this topic has increased, the understanding of some
fundamental concepts has sometimes been overlooked. Many papers have sited some, “supposed
microwave effects,” 41 that have been dismissed as just efficient rapid thermal heating. 42- 47 Some
have boldly dismissed any non-thermal rate enhancement effects completely. 38
The reasoning behind most of these claims is that previous work in microwave-assisted
organic reactivity was unaware of a fundamental flaw in their temperature reading. Many
researchers had relied on a temperature reading that underestimated the temperature in the
microwave, and this temperature difference was enough to account for the supposed rate
enhancement.
More specifically, this error was made due to the assumption that a solution cannot
superheat if ambient pressure and reflux conditions are met
37
35,36
. In a set of experiments by
Chemat,40 large temperature differences in the microwave when compared to thermal
temperatures were observed at reflux conditions. These sets of experiments have proven the
reflux temperature assumption to be false; however, these results may have left some questions
unanswered. The goal of this research was to not only confirm these results, but to improve upon
the methodology of Chemat’s experiments, using an IR pyrometer to directly measure the
solution, instead of an IR sensor measuring the quartz microwave vessels.
40
Also we aimed to
provide a set of reaction conditions that could provide standardized reproducible temperature
profiles when using microwave heating.37
3.2 Results and Discussion
3.2.1 Reflux Conditions
Various experiments were run to replicate previous reflux experiments by Chemat.40 In
our measurements the maximum delta T was higher than previously reported. Figure 3.1 shows a
variety of solvents run with a quartz vessel. These results were coupled with another set of
experiments that were done using a PTFE tube and the same methodology, much higher
temperatures were achieved when using the PTFE tube due to the lack of nucleation sites when
compared to the quartz vessel trials.39 These experiments show that under reflux conditions in a
quartz vessel superheating is not only present, but is drastic after 3 minutes of fixed wattage
experiments.
38
Temp (oC)
Still 75 W
110
100
90
80
70
60
50
40
30
20
Methanol
2-Propanol
Ethanol
Bp Methanol
Bp 2 Propanol
Bp Ethanol
0
50
100
150
Time (s)
Figure 3.1 Experiments run with no stirring present under reflux conditions, at 75 W for
3 minutes. The boiling points of each solvent have been plotted for clarity.
3.2.2 Stir Bar Effects
Stir bars act as nucleation sites for boiling to occur in microwave vessels. However the size
of the stir bars show enough difference that it is worth investigating which stir bar will give
reproducible temperature profiles. The smaller stir bars show profiles where one could observe
up to 5oC of temperature variance. This is significant enough to cause inflated rate
enhancements that may not be reproducible. Figure 3.2 shows this more erratic temperature
profile when using a typical 10 x 3 mm stir bar.
39
IPA Stir 75W
90
88
IPA
Run 1
86
Temperature (oC)
84
IPA
Run 2
82
80
IPA
Run 3
78
76
Bp IPA
74
72
70
0
200
400
600
Time (s)
Figure 3.2 Small stir bar (10 x 3 mm) with stirring on under reflux conditions at a fixed
wattage of 75 W for 6 minutes.
The medium sized stir bar (25 x 5 mm) was used with the same solvents and compared to
see if the reflux superheating temperature profile was improved. It was observed to be a much
more stable temperature profile, and provided reproducible temperature plots. This should
clearly show how nucleation sites can provide a reduced superheating temperature. Figure 3.3
shows this result over a 6 minute run. The IPA was not the only solvent that saw this effect. The
different solvents all seemed to behave in similar heating profiles. The most violent solvent
heating profile was the methanol that was not shown due to the irreproducibility of the study.
The IPA was used to show a typical solvent behavior that was indicative to the entire sample.
40
IPA Stir 75W
90
88
86
84
Temperature (oC)
82
80
78
76
74
72
70
0
100
200
300
400
500
600
Time (s)
Figure 3.3 Medium stir bar (25 x 5 mm) experiment with stirring on under reflux
conditions at a fixed wattage of 75 W for 6 minutes.
When no stir bar is present in the vessel, the temperature difference when compared to the
boiling point is very large. In ethanol, the temperature increase was over 24.4% greater than the
boiling point when no stirring was present. When the stir bar was included in the solution it
yielded a 3.8% increase in temperature. Figure 3.4 shows this result over a 3 minute window.
This effect is not only drastic, but it happens after only 2 minutes when using a power of only
75W. The stir bar also showed more reproducibility as expected when compared to the still
solution.
41
Figure 3.4 Ethanol without a stir bar plotted against a stirred experiment, the boiling
point was also shown for 3 minute run.
When replacing the organic solvents in the microwave reactor, reproducibility also
varied. Specifically figure 3.5 shows methanol with and without a stir bar. Although no one
would opt to use no stirring in an organic synthesis, this does show that irreproducibility is
somewhat correlated with specific solvents. Some samples may experience less predictable
heating curves which could lead to different rate enhancements from run to run. An important
note for all of the samples run is that they did not see much of an effect on the superboiling of
the sample when measured by a fiber optic probe. The fiber optic probe would, in theory,
increase the nucleation sites in the sample. The superboiling instead was mitigated by only about
1oC when measured comparatively with an IR pyrometer.
42
Figure 3.5 Methanol without stirring plotted against a stirred experiment, the methanol is
much more chaotic when compared to the ethanol.
Stir 75W
110
Methanol
100
2-Propanol
90
Ethanol
Temp(oC)
80
70
Bp
Methanol
Bp 2Propanol
Bp Ethanol
60
50
40
30
20
0
20
40
60
80 100 120 140 160 180
Time(s)
Figure 3.6 Small stir bar experiment for different solvents over 3 minute period.
43
Even when using stirring and a commonly used stir bar (10 x 3 mm) variance is clearly
seen when observing the heating profile. Figure 3.6 shows three different solvents and the
irregular superheating that is observed. This oscillation can result in data that would be hard to
reproduce in the microwave; even though this was run in a quartz tube at 75W under reflux
conditions and stirring was present. This range of 4o to sometimes 8o is the most troubling part of
reporting reflux temperatures in organic reactions in the microwave.
3.2.3 Nucleation Site Effects
Some other experiments were done with ground quartz chips that were then added to the
reaction vessel along with the medium stir bar. These results kept the superheating delta T to 12o C, this proved to be the most effective way to mitigate superheating in the microwave.
Changing the condensers had little to no effect on the temperature profile. The final parameters
tested were the vessels diameters and the volume of the solution. The volume resulted in more
unpredictable profiles at lower volumes. The 5mL solvent superheated more violently and
quickly when compared to the 30mL study. The diameter of the PTFE tubes was modified to
probe vessel effects because of the small amount of nucleation sites on the PTFE tubes. These
results showed that the larger the tube, the more stable the temperature profile. Obviously these
results are similar to the volume effects, as the microwave system has an optimal height for an
exposed solution. The trend was that some smaller vessels showed more erratic superheating
profiles. The fiber optic results matched closely with the IR temperatures as the Teflon tip was
smooth and didn’t provide enough of a nucleation site for the organic solvents.
44
3.3 Experimental Section
A CEM Discover (R) SP2 2.45 GHz microwave system was used in all experiments. A
standard quartz and Pyrex test tube were used in most experiments to compare to a typical
literature experiment. Additionally, a modified PTFE tube was used to image some samples from
the side of the microwave. These tubes were used as a control experiment to verify proper
stirring in the solvents, as our temperature readings were from the surface of the solvent (PTFE
is IR transparent in the range our pyrometer reads temperature which allowed reliable
temperature readings through the thin tube). The reaction conditions that were used were 30mL
of organic solvent, at a fixed wattage of 75W. The stir bars were changed according to desired
conditions, and the reflux conditions were all consistent for each run. All stirring experiments
were run with stirring on the highest setting possible in the CEM system.
Figure 3.7 Side angle of superheated IPA during a typical run, this image was taken
using the PTFE tube instead of the typical quartz vessel to obtain the image.
45
Temperature was recorded using a NIST calibrated Omega OS550A (R) IR pyrometer.
The optics of the pyrometer were optimized for distance to the surface of the solution with a spot
size diameter of 10mm. The PTFE experiment was recorded using a FLIR TG165(R) IR imaging
camera. These surface temperatures were calibrated to the IR pyrometer used for the rest of the
investigation. The fiber optic studies were conducted using a Neoptix (R) fiber optic temperature
probe.
The IR pyrometer experiments were used to verify varied sizes of the PTFE tubes heating
properties. A hole on the side of the microwave was used to view the sample from the side seen
in figure 3.7. The temperature of the organic solvent was measured by using the standard
emissivity of .95 which is used for water and other organic solvents. The temperature image was
plotted using an average of the center of the solution in the PTFE tube that was exposed to the
microwave field. Tracking the temperature change over time was used to verify measurement
sensitivity with the other temperature recording methods.
3.4 Conclusions
Table 3.1: Difference in Temperature for Solvents and the Parameters Explored
46
Table 3.1 shows the delta T for the different parameters explored. More solvents have been
studied; however, these specific solvents show the trend for superheating in reflux conditions.
From these results it is clear superheating under reflux can potentially provide a large shift in the
rate enhancement for many microwave relevant organic reactions. It is also clear that when the
conditions are optimized by using larger stir bars a quartz tube and quartz boiling chips,
superheating is nearly completely mitigated. It should be noted that when reporting temperatures
it may be better to report them at a 3o C error, which is temperature difference between the
boiling point when running standard reflux reaction conditions without quartz boiling chips.
Based on our experiments, we conclude that the published literature accounts of microwaveassisted superheating temporary elevated temperatures are achieved under microwave irradiation.
A system must have sustained nucleation points that cannot usually be achieved by just using stir
bars. Nucleation sites should be increased by using boiling chips when possible to remove the
risk of superboiled solutions. An unstirred microwave reaction should always be considered a
superheated solution even at “reflux conditions.” Understanding the magnitude of a systems
superheated state is dependent on a number of factors: pressure, volume of vessel, nucleation
sites in the system, and the concentration of the microwave active organic material. These factors
must be considered when trying to control bulk solution temperature under a given set of
experiments conditions. It is clear that some of the microwave-assisted organic chemistry
literature needs to be looked at again, but many of the results that were deemed invalid should
also be explored again.
47
CHAPTER 4
A MECHANISTIC STUDY OF SOLID-GAS MICROWAVE
ENHANCED REACTIONS: NITROUS OXIDE CONVERSION
OVER GRAPHITE
4.1 Introduction
As discussed in chapter 2 we have shown that the use of microwave radiation to drive
highly endothermic gas-carbon equilibria such as the Boudouard and steam-carbon reactions
results in dramatic shift in the position of the equilibrium towards the product side. This
observed shift in the equilibrium, which corresponds to an apparent exothermic shift in the
enthalpy of the reaction, was attributed to the microwave-specific enhancement in the forward
reaction rate.
In those studies we hypothesized that there are two places in the mechanism for these
reactions where microwave radiation could enhance the forward reaction (figure 4.1 a).
A
B
Figure 4.1 (a)Hypothesized mechanism for microwave enhancement of the steamcarbon (shown in black) and Boudouard (shown in red) reactions. (b) proposed
mechanism for increased CO generation during Boudouard reaction.
48
Specifically figure 4.1 b shows the possible mechanism from which increased reactivity was seen
during the Boudouard reaction. This specific process was the mechanism of interest when
choosing the nitrous oxide graphite system. The preliminary pilot study showed that the
microwave reaction was made up of significantly more carbon monoxide when run at similar
temperatures. The primary dielectric loss process that results in heating of the carbon is spacecharge recombination (as described in chapter 1). In this mechanism, oscillations of the electric
field induce charge migration in the material. The trapping of these charges at defect and
impurity sites hinders their recombination, which defines part of the loss process and that leads
to the generation of heat. The presence of charged sites on the surface makes it more reactive
towards the initial oxidation process (Figure 4.1a), thereby making that process more efficient.
The second way that microwaves can accelerate the forward reaction is through selective, Debye
type heating of the polar surface oxide group. This will accelerate the rate-determining step of
CO elimination from the surface (Figure 4.1b).
4.2 Carbon-Nitrous Oxide Reaction
While these hypothesized microwave-specific interactions are reasonable and consistent
with known dielectric loss processes they are difficult to experimentally verify.48-50 In this study,
the comparative kinetics of the reaction of N2O with carbon under microwave and conventional
convective (henceforth referred to as “thermal”) heating is used to understand better the
mechanistic origins of microwave enhancement of the reaction.
The reaction of N2O with carbon produces N2, CO2 and CO (eq 4.1,4.2). Under most
reaction conditions the primary oxide of carbon produced is CO2, which dominates, in part, due
to its extreme exothermicity. In fact, most studies of the reaction consider only the production of
CO2 as CO is either not detected or is present in insignificant quantities.
49
2N2O + C  CO2 + 2N2
ΔH = -951.12 kJ/mol
Eq 4.1
N2O + C  CO + N2
ΔH = -192.58 kJ/mol
Eq 4.2
Mechanistically the reaction parallels other gas-carbon processes such as the Boudouard (CO2 +
C  2CO) and steam-carbon (H2O + C CO + H2) reactions in that the initial process is the
oxidation of an available carbon site at the surface and ejection of, in this case, nitrogen (rxn.
4.3).
Eq 4.3
Eq 4.4
Eq 4.5
Eq 4.6
The production of carbon dioxide arises primarily from the secondary reaction of N 2O with the
surface oxide site (rxn. 4.4). The production of carbon monoxide occurs through the ejection of
CO from the oxidized surface (rxn. 4.5). This process, which is rate determining in the
Boudouard and steam-carbon reactions, is slow and does not compete with rxn. 4.4, the N2O
surface oxide reaction (i.e. k2≫k4), which is why CO is, at best, a minor product. Another
potential source of CO2 is the reaction of CO with the oxidized surface (rxn. 4.6). This is the
reverse of the Boudouard reaction and, hence, is thermodynamically favorable, but it will likely
be relatively inefficient due to the low concentration of CO present.
While the decomposition of N2O over carbon and other catalysts is of interest as a method
of remediating NOx pollutants, our interest in this reaction is as a mechanistic probe of
50
microwave specific rate effects. In particular, this reaction generates a distribution of distinct
products through different mechanistic steps that can potentially be affected by the radiation. If
the proposed mechanisms of microwave enhancement of gas carbon reactions were correct, then
the rate at which specific products are generated and, therefore, the overall distribution of
products, of the N2O carbon reaction would be affected in predictable ways under microwave
radiation.
Figure 4.2 Hypothesized microwave effects the N2O-carbon reaction due to spacecharge recombination and selective activation of the surface oxide.
In particular, if the existence of radiation induced space-charges does accelerate the initial
oxidation process (figure 4.2a) we would expect a more efficient production of N2 with lower
apparent activation energy, than is observed under conventional convective heating. Similarly,
the production of CO, which is a minor product under conventional thermal conditions, will be
expected to be higher under microwave conditions if, as is proposed, Debye-type loss processes
cause selective heating of the polar surface oxide group which facilitates their ejection as CO. As
such, this reaction is uniquely suited for separating specific microwave effects and determining
their magnitude.
51
4.3 Results and Discussion.
The reactions were carried out in a flow reactor with N2O passing over the graphite at a
constant rate at a pressure of approximately one atmosphere. The temperature of the graphite
surface was measured directly by means of a pyrometer focused at the surface through a
germanium window. The N2O carbon reaction is exothermic, which manifests itself through a
marked increase in temperature of the graphite when the reaction is proceeding. Figure 3 show
the temperature of the carbon under 30 W of applied microwave power. Prior to the introduction
of N2O the carbon rapidly heats then starts to plateau at ~306 °C and begins to approach a steady
state value. Commencement of N2O exposure resulted in an abrupt jump in temperature to ~363
°C, which ultimately reached a steady state value.
Figure 4.3 Temperature of graphite and nitrous oxide reaction under 30 W of
microwave irradiation. The exothermicity of this reaction is clearly shown with the large
temperature spike, which is correlated with the production of N2 gas.
52
4.3.1 Product Distribution
The distribution of products generated from the reaction of N2O with graphite under
convective and microwave heating conditions is shown in Figure 4.4. As can be seen, the product
distribution differs profoundly different microwave and convective heating over a similar
temperature range.
Figure 4.4 Composition of product gas stream as a function of temperature: comparing
the percentage of product species between the two processes.
Under convective heating, very little reaction is observed at the beginning temperature of
504 °C with only about 16% of the N2O consumed. It should be noted that we investigated the
composition of the thermal experiments at temperatures lower than 504 oC and the results were
nearly identical. However, between 504 °C and 616 °C, which was the next temperature
investigated, there is a dramatic jump in the extent of the reaction with approximately 90 % of
the N2O is consumed. This suggests a temperature threshold for the reaction above which the
reaction becomes very facile. As the temperature was increased, the overall extent of reaction
also increases steadily increased resulting in approximately a 98% conversion of N2O at 659 °C.
Interestingly, at the highest temperature investigated, 669 °C, the reaction appears to be
somewhat inhibited with N2O consumption decreasing to 93%. While the inhibition is
53
reproducible and well within the standard error for the composition determination, it’s origin is a
little unclear but likely arises from opposing temperature effects in the rate constants of
mechanistic steps of the reaction. Consistent with prior studies of this reaction, very little CO is
produced (<1%) at any temperature with the primary products being N2 and CO2.
A dramatically different product distribution is observed at comparable temperatures
under microwave irradiation. The reaction is much more facile, with about 80 % of the N2O
consumed at the lowest power. The extent of the reaction increase with applied microwave
power and the temperature of the graphite. As the power increases the extent of the reaction also
increases until at 70 W corresponding to a graphite temperature of 559 °C virtually all of the
N2O is being consumed. Significantly, the amount of CO produced is much greater under
microwave conditions and, while never the dominant product, accounts for 13% of the product at
70 W. This provides strong support for the hypothesis that microwave-specific coupling of the
radiation to the surface oxides will accelerate dissociation of CO from the surface (eq 4.5). The
increased formation of CO will be at the expense of CO2 as it will deplete the concentration of
surface oxides making eq 4.4 less efficient.
4.3.2 Reaction Kinetics
For the mechanistic steps above (eq 4.3-4.6), rate expressions can be derived using a
steady-state approximation for the surface oxide similar to the previous chapter. The rate of N2
appearance is given in terms of the partial pressure of the reactant by eq 4.7, where the rate
constants are defined in eq 4.3-4.6, C* is the active site concentration on the carbon surface.
rN2 = k1 pN2OC * + k2 pN2O
k1 pN2OC *
k2 pN2O + k3 + k4 pCO
æ
ö
k2 pN2O
= ç 1+
k1 pN2OC *
k2 pN2O + k3 + k4 pCO ÷ø
è
54
1.)4.7
Eq
If we assume that k3≪k2, which is reasonable since it represents the rate determining step, the
rate expression can be approximated by eqn. eq 4.8.
æ
ö
ç
÷
pN2O
rN 2 » ç 1+
÷ k1 pN2OC *
k4
ç
pN2O + pCO ÷
k2
è
ø
Eq2.)
4.8
Finally, for the thermal reaction and the microwave reactions at lower powers, the production of
CO is quite small so that pco~0. Since the partial pressure of the reactant is constant (pN2O~1), the
rate expression reduces to an approximate zero-order expression given in eq 4.9.
rN2 » 2k1 pN2OC *
3.)
Eq 4.9
Experimentally, the rate of N2 evolution over time follows zero-order kinetics well. Apparent
rate constants over a range of carbon temperatures under both conventional convective and
microwave heating conditions were determined.
Figure 4.5 Arrhenius plot of the thermal and microwave reactions.
55
4.4 Conclusions
Arrhenius plots of the convective and thermal kinetics data are shown in Figure 4.5. As
can be discerned from the dramatic difference in slopes, the apparent activation energy of the
thermal and microwave driven process differ dramatically. The apparent activation energy,
determined from least squares analysis of the plots, is 114.7(±5.6) kJ/mol for the thermal
reaction while the microwave driven reaction is nearly an order of magnitude lower at 15.2(±0.6)
kJ/mol. Clearly, the first step in the process, the oxidation of the surface and ejection of N 2 (eq
4.3) becomes much more facile due to a specific microwave interaction.51-53 Obviously, the
kinetic results do not allow us to identify specifically the origin of the effect, but the results are
consistent with enhanced reactivity of the surface due to charge-separation loss processes (Figure
4.2a). Alternatively, if molecular N2O adsorbed on the surface has any appreciable residence
time, it could couple to the radiation and undergo Debye-type heating that could result in rapid
dissociation of N2. Regardless, the effect of the microwave is to change the thermo-kinetic
parameters of the initial reaction.54 The immediate consequence of this is that there will be a
higher degree of oxide formation on the surface, which can impact the secondary reaction that
account for their products.
56
CHAPTER 5
CONCLUSIONS
In microwave-assisted chemistry, temperature measurements have been critical to
understanding thermal chemical dynamics. Recently, claims have been made to disprove entire
bodies of work due to incorrect measurements. In chapter 3, discussion on this controversy
should clearly show that such claims must have more evidence to completely disregard
microwave effects. In our systems we have verified that our temperature measurements have
been done correctly by using many different methodologies to read temperatures (IR Camera, IR
pyrometer, IR sensors, and Near IR). We have also shown that our gas-solid reaction systems
have some innovative apparatus designs that allow for a multitude of different measurements on
these solid-gas reactions. Using G.C. and an IR pyrometer we have begun to explore some of the
older gas-solid chemistry systems that are still industrially relevant today.
Heating of solids in the microwave undergoes the general Maxwell Wagner interfacial
polarization mechanism which has become the recognized mechanism for understanding the
heating of carbon materials. It has been proposed that the physical surface phenomena in these
systems have often led to unique microwave effects, including enhancements of some reactions.
Gasification of carbon has been a long standing industrially relevant area in chemistry, and this
research may provide a new prospective for this process in the future. Increased reaction kinetics
and overall energy savings from selective heating of the solid catalyst in the microwave field
could be key to the innovation of this process. Specifically, the steam-carbon and the Boudouard
reactions discussed highlight these potential advantages and energy savings. These largely
57
endothermic processes have shown to have large reductions in apparent activation energies when
under microwave irradiation. These reductions in energy cost could lead to new technologies in
the gasification sector, but more importantly, understanding why these activation energies are
lowered could more broadly help the field innovate their processing.
The nitrous oxide-carbon reaction was key to understanding the mechanism more
intimately. Most surface reactions involve diffusion followed by adsorption. Manipulations of
active sites may play a key role for the surface adsorbates reactivity. The generation or
elimination of an active site may mechanistically help increase or decrease desired kinetic rates.
Going forward, controlling the desired surface phenomenon, either by the amount of adsorbed
species through surface defects and surface area, or by controlling the active sites of the surface,
through charged electron hole pair species, may be an area that is critical to more controlled gassolid reactivity.
A more significant, largely endothermic system, such as water splitting may also be a
target area of study. By using microwave irradiation, microwave active spinnels, or other metal
oxide catalysts may finally reach reasonable reaction conditions to see innovation in this
somewhat static field of chemistry. By understanding previous surface phenomena we can better
target systems that we see as good candidates for energy savings by increasing active sites on the
surface.
58
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BIOGRAPHICAL SKETCH
During his undergraduate years, Anthony Ferrari worked in the Hunting Lab at Ithaca
College doing solid-state inorganic chemistry, synthesizing conductive metal oxides for
application of a cathode support in fuel cells. In spring of 2011 Anthony graduated from Ithaca
College and began work in the fall in the doctoral program at Florida State University in the
department of Chemistry and Biochemistry. Anthony worked under Dr. Albert Stiegman, and
has worked in collaboration with a number of different groups who are also driven by the
emerging microwave chemistry that has been a new focus on at Florida State University.
63
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